Um. If you are not an artist, why would you be coloring art? It's not like coloring is easier than drawing. (How you would make an RTS work in Flash without massive lag is a whole other question.) I mean, you could actually take someone's pencil or ink drawing and scan it in and then color it yourself in Photoshop or Gimp. You'd want to use "blending options: multiply", google for a tutorial. Or if you are aiming for a relatively simple cartoon style coloring you can scan the drawing and import it to a vector program such as Illustrator or inkscape. Then you either trace over the lines by hand or use the auto-trace and then clean it up a bit by hand, and use the lines to create areas of color fill. Still wouldn't look like much unless you know how to apply some highlights and shadows to it. A building you probably wouldn't be animating, but vector art is Flash's native format and can be more flexibly animated within Flash than bitmap/raster/pixel art can. Using vector art within a game makes the game scale really well to different resolutions and zooms, but can be costly performance-wise.

Ah, actually that is much like the game I am currently making art for, it's a 2D isometric game where you breed monsters in the sim portion of the game and use them as units in the tactical combat portion of the game. I personally no longer bother drawing most things with pencil and paper, but I did when I was still getting started with digital art. It is possible to get a servicable piece of game art by scanning a drawing and digitally coloring it, or by scanning something already colored or taking a high resolution photograph of a painting or even a real objects. On the other hand it is also possible to draw directly into 2d art software via a tablet or a mouse, which most experienced digital artists do because it is more efficient and faster. Many art students will be in the process of learning how to do this, so some would have already learned enough to be able to provide you colored digital images as files (usually png) which you then use in flash just like the batch of tutorial images that come with flash.

But personally, if I were you, since you don't have any experience with the art production aspect I think you will have a lot less frustration and confusion if you hire someone who has a bit of experience making images for games. In a case like that all you'd really have to do is provide the artist with an isometric template of appropriate size (in pixels), tell them what you need an image of, and provide them with a few sample images of the style you want your game art to have. Then they can just email you a sample, and if it's good you agree on how many similar images you want and what you are paying for them.